214 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



early stages in the formation of the stolon of the primary ascidiozooid 

 of pyrosoma (17, pp. 50-56), says that Seeliger has failed to find the first 

 stages, and he himself gives an account of the origin of the germinal 

 mass which agrees with my own observations on salpa in all the points 

 of disagreement between my own account and Seeliger's account of its 

 origin in pyrosoma and salpa, for he shows that it can be recognized as 

 a definite mass of cells before the stolon itself is formed ; that neither the 

 nerve-tube nor the perithoracic tubes are derived from it, and that it 

 gives rise to the reproductive organs and to them alone. He says 

 (page 52) : " Long before the stolon begins to project, the group of fixed 

 mesenchyma cells which forms the first rudiment of the genital rod 

 makes its appearance on the lower wall of the subintestinal blood sinus. 

 Thus the point where the stolon is afterwards produced is designated. 

 Neither in surface-views nor in sections can we find, at this time, any 

 projection from the outer wall of the body of the ascidiozooid to indicate 

 the existence of a stolon." 



SECTION 3. -The Differentiation of the Germinal Cells. 



At first the germinal mass is homogeneous, Plate XX, Fig. 6, and 

 consists throughout of embryonic cells which are uniform in character 

 and which do not present any marked difference from the cells of blasto- 

 dermic origin in other parts of the embryo. Soon differences appear among 

 them, and as the stolon grows and the germinal mass becomes elongated, 

 the amount of differentiation becomes greatest at the extremity, as 

 shown in Plate XVI, Fig. 5 ; while the proximal end retains its embry- 

 onic, undifferentiated character. The changes may therefore be studied 

 either in a series of embryos, or in a series of sections through different 

 parts of a stolon which is somewhat advanced in development. 



I have employed both methods, and I give figures of sections from a 

 series of stolons, and also a series of successive stages from base to tip of 

 each stolon. 



Plate XX, Figs. 5, 7 and 6, are longitudinal vertical sections through 

 very young stolons. Plate XVI, Fig. 5, is a longitudinal section of an 

 older stolon. Plate XX, Figs. 1, 2, 3 and 4, are transverse sections 

 through a stolon a little older than the one shown in Fig. 7 ; from an 

 embryo like the one shown in Plate XLI, Fig. 3. Plate XVI, Fig. 5, is a 

 longitudinal section of a stage somewhat older, and the figures on 

 Plate XXI are successive sections, from base to tip, of a stolon from an 

 embryo like the one shown in Plate XLI, Fig. 5. 



